"You can safely say that 2011 was very atypical: In over 30 years of satellite records, we hadn't seen any time where it was this cold for this long," study researcher Susan Strahan, an atmospheric scientist at NASA Goddard Space Flight Center, said in a statement.
Using atmospheric simulations, Strahan and her colleagues found that a mix of cold temperatures, chlorine and an unusually strong Arctic vortex caused the odd thinning. The Arctic vortex is a region of fast-blowing circular winds that get stronger each fall, creating an eddy of chilled air around the pole.
In 2011, the atmosphere was unusually quiet, allowing the Arctic vortex to remain strong well into the spring, after it usually breaks up. The reappearance of the sun in March while it was still especially cold created the conditions that led to the ozone thinning, the researchers report in the Journal of Geophysical Research - Atmospheres.
"Arctic ozone levels were possibly the lowest ever recorded, but they were still significantly higher than the Antarctic's," Strahan said. "There was about half as much ozone loss as in the Antarctic," and the levels remained above the threshold for calling the ozone loss an actual "hole," Strahan added.